1. Field of the Invention
This invention relates to the field of acrylic polymers and more particularly, to processes for preparing all-acrylic copolymer latices which are useful as coatings, finishes, adhesives, and the like.
2. Description of the Prior Art
Acrylic latices are emulsions of acrylate comonomers, often including minor amounts of other comonomers as, for example, acrylic acid or methacrylic acid. The procedures used to prepare the emulsions are well known in the art and generally involve admixing under rapid stirring the comonomer charge with water, surfactants or emulsifying agent, a polymerization catalyst or initiator, and, in most cases, a protective colloid-forming substance. The mixture is then heated to polymerization temperature for the time required to substantially complete the emulsion polymer formation. Many variations of the general preparative procedure are used to obtain, for example, lower viscosity, high solids content, improved freeze-thaw stability, etc. These variations include, for example, the use of a pre-emulsion technique (see U.S. Pat. No. 3,637,563) in which the monomer charge is first emulsified in water using a surfactant or emulsifying agent, and then the pre-emulsion is added to the reaction mixture which contains the remaining reactants; intermediate addition of surfactant to the polymerization reaction mixture during the polymerization (see U.S. Pat. No. 3,423,352); and addition of a water-soluble free radical initiator to a substantially completely polymerized emulsion and reacting further to reduce the viscosity of the emulsion (see U.S. Pat. No. 3,423,353).
The copolymerization of all-acrylic comononers employing known and conventional acrylate catalyst systems, for example, redox catalysts such as sodium metabisulfite-potassium persulfate, frequently proceeds sluggishly due to unfavorable reaction rates. While it is known that the introduction of small quantities of multifunctional acrylate monomer into vinyl-acrylic copolymers (commonly assigned copending U.S. patent application Ser. No. 856,363 filed Dec. 1, 1977) and vinyl-acrylic copolymers (commonly assigned copending U.S. Pat. No. 4,056,503, filed Oct. 18, 1976) will significantly improve the properties of these coating resins, heretofore there has been no recognition or appreciation that the use of minor amounts of multifunctional acrylates will dramatically accelerate the copolymerization of all-acrylic latices.